Took voltage measurements of the smaller filter caps, and here is the results

250VDC & 10mVAC (Should be 300VDC, but I have a 100W bulb in line with the power cord.)

170VDC & 15mVAC (Should be 200VDC, but I have a 100W bulb in line with the power cord.)

Can't measure the main 220mfd 300V caps because I can't reach the other side of the caps unless I desolder a bunch of wires and pull the board and jumper everything back in thanks to the MB's terrible design.

I don't think I really want to do the repair, but I do want to at least find the fault.

I checked all the power tubes and preamp tubes by putting them in my Epiphone Valve Junior. I tapped on each tube (both power and preamp) with a pen, and no static or ringing noises. In short, all the tubes work.

I checked all the power tubes and preamp tubes by putting them in my Epiphone Valve Junior. I tapped on each tube (both power and preamp) with a pen, and no static or ringing noises. In short, all the tubes work.

Looking at the schematic, it seems the speaker switch disconnects the preamp from the power amp.. Not the effects loop. So I could solder a wire to the input of the power amp and see if music makes it through cleanly, and isolate the issue? Or am I crazy?

In fact switching the SILENT RECORDING switch to MUTE cuts all noise, but it doesn't appear to disconnect the speaker or place a dummy load. Instead it looks like it is breaking the connection connecting the preamp to the power amp.

The manual says "This switch mutes all of the signal at the power section driver stage, removing the need for a speaker load IN THIS POSITION ONLY!

So you can run a tube amp w/o a load on the output transformer provided you do NOT have an input source? (in this case, the preamp)

ouimetnick
after checking out your you tube vid and the schematic and my failing memory i would be suspicious of the relays and their supply the fact that you get no sound on one channel and the weird fart noise on power down i remember this as clues in previous repairs that lead me to replacing relays only to discover bad zeners and bad 16 v filters in the switching matrix supply

ouimetnick
after checking out your you tube vid and the schematic and my failing memory i would be suspicious of the relays and their supply the fact that you get no sound on one channel and the weird fart noise on power down i remember this as clues in previous repairs that lead me to replacing relays only to discover bad zeners and bad 16 v filters in the switching matrix supply

one of the relays is channel switching the other is selecting which channel the reverb is applied to.
i was wondering do you have the footswitch if i recall this amps lead channel won't work without it.
or was it something about push pull controls being set right, might be another model i'm thinking of ...

the matrix circuit in these can be finicky
did you in fact replace all the diodes because not all are the same type! which ones did you replace?